G.2 Tier II - Theoretical Bioaccumulation Potential (TBP) Of Nonpolar Organic Chemicals

The TBP calculation assumes that various lipids in different organisms and

organic carbon in different sediments are similar and have similar distributional

properties. Other simplifying assumptions are that chemicals are freely

exchanged between the sediments and tissues and that compounds behave

conservatively. In reality, compound size and structure may influence

accumulation, and portions of organic compounds present on suspended

particulates may have kinetic or structural barriers to availability. Another

important assumption implicit in the TBP calculations is that there is no

metabolic degradation or biotransformation of the chemical. Organic-carbon

normalized contaminant concentrations are used such that the sediment-

associated chemical can be characterized as totally bioavailable to the organism.

Calculations based on these assumptions yield an environmentally conservative

TBP value for the dredged material if the dredged material in question is the only

source of the contaminant for the organism. However, note that TBP calculations

are not valid for sediments with TOC less than or equal to 0.2 percent.

McFarland (1984) calculated that the equilibrium concentration of nonpolar

organic chemicals, which the lipids of an organism could accumulate as a result

of exposure to dredged material, would be about 1.7 times the organic carbon-

normalized concentration of the chemical in the dredged material.

Concentrations are directly proportional to the lipid content of the organism and

the contaminant content of the dredged material or reference sediment, and are

inversely proportional to the organic carbon content of the dredged or reference

material (Lake, Rubenstein, and Pavignano 1987).

The possible chemical concentration in an organism's lipids [the lipid bio-

accumulation potential (LBP)] would theoretically be 1.7 times the concentration

of that chemical in the sediment organic carbon. Rubinstein et al. (1987) have

shown, based on field studies with PCBs, that a value of 4 for calculating LBP is

appropriate. LBP represents the potential contaminant concentration in lipid if

the sediment is the only source of that contaminant to the organism. It is

generally desirable to convert LBP to whole-body bioaccumulation potential for

a particular organism of interest. This is done by multiplying LBP by that

organism's lipid content, as determined by lipid analysis or from reported data.

Soft-bodied animal lipid contents may range from 1 to 2% wet weight (based on

data from an oligochaete, midge, and amphipod species.1

Based on work by McFarland and Clarke (1987), TBP can be calculated

relative to the biota sediment accumulation factor (BSAF) as:

TBP = BSAF (Cs / %TOC) %L

where TBP is expressed on a whole-body wet-weight basis in the same units of

concentration as Cs, and

G. Angley, Environmental Protection Agency, Duluth, and H. Lee, EPA, Newport,

personal communication.

Appendix G Animal Bioaccumulation Test Procedures